With the growing prevalence of wind farms, conflicts are increasing between wind farm locations and electromagnetic radiations from radar systems. The USA Department of Energy's goal is to be generating 30% of the country's electricity using renewable energy sources by 2030. This will necessitate installation of thousands of wind turbines, exploiting the rich areas across the globe. However, around 20 GW of the potential wind energy is not accessible because it lies in the range of navigational radars. Wind turbines are large electromagnetic radiation reflectors and affect the radar operations. Reflection of radar from components of wind turbines causes several problems such as “radar systems used to monitor aviation traffic cannot easily discriminate between moving turbine blades and aircraft”. Although towers and other components are stationary, they also cause problems. Their presence can be distinguished from aircraft, but they create a shadow zone, where presence of aircraft can be difficult to identify. Next table explains the wavelengths used by the different radar systems.
FrequencyWavelengthBandRange (GHz)RangeUsesVHF50-330MHz6 m to 90 cmVery long-rangesurveillanceUHF300-1,000MHz1 m to 30 cmVery long-rangesurveillanceL1-2GHz30 cm to 15 cmLong-range surveillance,en route traffic controlS2-4GHz15 cm to 7.5 cmModerate-rangesurveillance, terminaltraffic control,long-range weatherC4-8GHz7.5 cm to 3.8 cmLong-range tracking,airborne weatherX8-12GHz3.8 cm to 2.5 cmShort-range tracking,missile guidance, mapping,marine radar, airborneinterceptKu12-18GHz2.5 cm to 1.6 cmHigh resolution mapping,satellite altimetryKa27-40GHz1.1 cm to 0.75 cmVery high resolutionmapping, airportsurveillance
Among these bands S and X band frequencies are of prime importance to the wind turbine radar cross section (RCS). There is a need to reduce the RCS of the wind turbines in S and X bands to avail the wind energy in this region. Reducing RCS of the turbine blade is the most critical task as the blade normally contains highly reflecting copper rod and has weight and shape constrains. Therefore efforts are focused on modification of wind turbine blades, so that they are invisible to the radar system. The accepted criterion for invisibility is attenuation of radar reflection/transmission up to −20 dB. Further, a robust design which could be implemented for a large scale of wind turbines is needed to install them in radar regions. Thus the design and composition of wind turbine blades should be less sensitive to variations in the manufacturing process and unavoidable errors.
There is known prior art to make invisible to radar wind turbine blades either by means of anti-radar coatings or using Frequency Selective Surfaces as described for example in WO 2010/122350. However, the prior art has not focused specifically to attenuation of radiation in the S and X bands which are those that most affect wind turbines.